1. Field of the Invention
The present invention relates to an image forming apparatus using the electrophotographic technology.
2. Description of the Related Art
An image forming apparatus such as a copying machine or a printer is required to match the output color with a target reference color. Hence, a technique of performing tone conversion so that the output density (color) matches a target density (color), based on the result of actually outputting an image, is widely used. To do this, the image forming apparatus executes tone (color matching) calibration. More specifically, first, the potential/exposure intensity is set such that the solid density matches a target value, and a plurality of patch images with different tones are then printed and output. Next, colorimetry is performed for these patch images to generate a reference tone conversion table according to which the output density (color) matches a target density (color). Printing an image upon correction using the reference tone conversion table allows output corresponding to a target reference density. Note that the plurality of patch images with different tones are printed without correction.
In an image forming apparatus which forms an image using toner particles, the amount of development, that is, the amount of toner used to form an image changes due to a change in charge amount of toner particles. This change adversely affects a wide range of tones. This effect becomes conspicuous especially in an image forming apparatus which uses toner particles and carrier particles as a developer, and fills an electrostatic latent image with toner in accordance with the charge amount of toner, thereby forming an image.
To suppress changes in output density (color), feedback control is also generally widely performed. More specifically, patch images are formed on an image carrier or a transfer body, and the densities of the patch images are measured by, for example, a photosensor, thereby performing control so that the output density (color) matches a target density (color). However, in feedback control, after the patch images are measured, a tone correction table is generated, and a tone conversion process is then performed for an input image, so a control time delay occurs. This makes it impossible to use feedback control to suppress fluctuations in density in a short cycle.
To suppress fluctuations in density in a short cycle, Japanese Patent Laid-Open No. 2001-42613 proposes a technique of estimating the charge amount of toner particles and controlling the contrast potential in image formation in real time.
Unfortunately, even when the above-mentioned image forming apparatus which performs tone calibration estimates the charge amount of toner and controls the contrast potential, the following problem is posed.
First, the amount of toner applied on a solid image is determined by the contrast potential and the charge amount of toner particles. More specifically, the amount of applied toner is proportional to the contrast potential and is inversely proportional to the charge amount of toner particles. Note that a change in developing capacity due to a temporal change of, for example, a developer need not be taken into consideration because it does not occur in the short term. Therefore, even when the contrast potential is set such that the amount of applied toner becomes appropriate by tone calibration, if the charge amount of toner particles then increases, the density lowers given a constant contrast potential. Note that if the increase in charge amount is large, it may be impossible to ensure a contrast potential which compensates for the decrease in density. Also, the conversion value for each tone of the reference tone conversion table depends on the contrast potential. Accordingly, when the contrast potential is increased to compensate for the decrease in density, the reference tone conversion table naturally shifts from an appropriate value. In this case, the error becomes large in a region having low to medium densities.